Add license info to README

Author: 4lDO2

README.md

@@ -12,3 +12,20 @@ type-erased async trait attribute proc macro, such as Dtolnay's `async-trait`
 crate, or a similar one. That said, it'd be awesome both for me and for the
 rust compiler if you could test this, find something that's missing in the
 compiler or here, and file an issue.
+
+## License
+
+Licensed under either of
+
+ * Apache License, Version 2.0
+   ([LICENSE-APACHE](LICENSE-APACHE) or http://www.apache.org/licenses/LICENSE-2.0)
+ * MIT license
+   ([LICENSE-MIT](LICENSE-MIT) or http://opensource.org/licenses/MIT)
+
+at your option.
+
+## Contribution
+
+Unless you explicitly state otherwise, any contribution intentionally submitted
+for inclusion in the work by you, as defined in the Apache-2.0 license, shall be
+dual licensed as above, without any additional terms or conditions.